Teeth in the wrong place
Why Do Chickens Swallow Stones?
A chicken has no teeth at all. It keeps a bag of small stones inside itself and grinds its food with those instead.
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Step 1 of 5
A beak cannot chew.
A beak is good at picking up, tearing and stabbing, and hopeless at grinding — there is nothing flat to rub together. So a seed goes down whole and hard.
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Step 2 of 5
First it is stored.
Food drops into a stretchy bag at the base of the neck called the crop. A chicken can fill it fast in the open and then digest slowly somewhere safer.
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Step 3 of 5
Then it is ground with stones.
The gizzard is a thick muscular pouch holding grit the bird has deliberately swallowed. It squeezes hard — a turkey's gizzard can crack a walnut — and after an hour or more the seed is milled between the stones.
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Step 4 of 5
The stones wear away and get replaced.
Grinding wears the grit smooth and small. A chicken picks up fresh grit constantly, which is why birds are so often seen pecking at bare ground and gravel paths.
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Step 5 of 5
No teeth keeps a bird light.
Teeth and the heavy jaw that carries them sit right at the front, where weight is worst for flying. Moving the grinding to the middle of the body is part of what makes a bird light enough to leave the ground.
Dinosaurs did the same thing. Smooth polished stones are found among their fossils, in exactly the place a gizzard would have been.
The short version
A chicken has no teeth, so it stores food in a crop and grinds it in a gizzard using stones it swallowed on purpose.
Try it yourself
Shake a few dry lentils in a jar with some gravel. The gravel does what the gizzard's grit does.